/Engine/Plugins/Animation/RigLogic/Source/RigLogicLib/Public/dna/layers/GeometryWriter.h
Both the mesh storage itself and it's face storage will be implicitly resized (if needed) to provide storage for the number of meshes and/or faces that are inferred from the specified indexes.
void setFaceVertexLayoutIndices ( std::uint16_t meshIndex, std::uint32_t faceIndex, const std::uint32_t * layoutIndices, std::uint32_t count )